Lutetia may have witnessed the birth of the Earth

Lutetia may have witnessed the birth of the Earth

When we look at the solar system now, we see it after it's had billions of years of evolution under its belt. Things have changed a lot since it first formed out a swirling disk of material, 4.5 billion years ago. We can make some pretty good guesses about the way things looked back then, though. We can see other systems forming around other stars, for example, to get an idea of what things look like when they're young.

But we can also look at our own solar system, look at the planets, the comets, the asteroids, and, like astronomical archaeologists, get a glimpse into our own cosmic past.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

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We know that asteroids formed along with the rest of the system back then. We also know that there are many kinds of asteroids: rocky, metallic, chondritic, some even have ice on or near their surface. Some formed far out in the solar system, and some formed near in. The thing is, we think the vast majority of the asteroids that formed close to the Sun were absorbed by -- and by that, I mean smacked into and became part of -- the inner planets, including Earth. Only a handful of those asteroids still remain intact after all this time. But now we think we've found one: the main belt, 130 km-long asteroid Lutetia.

Using a fleet of telescopes, astronomers carefully measured the spectrum of Lutetia -- including spectra taken by the European Rosetta space probe, which visited Lutetia in July 2010 and returned incredible close-up images (see the gallery below). The spectra were then compared to spectra of meteorites found on Earth -- meteorites come from asteroids after a collision blasts material from them, so they represent a collection of different kinds of asteroids that we can test in the lab here on Earth.

They found that the spectrum of Lutetia matches a very specific type of meteorite found on Earth, called enstatite chondrites. These rare rocks have a very unusual composition that indicates they were formed very near the Sun, where the heat from our star strongly affected their formation. They have a clearly different composition than meteorites which formed in asteroids farther out in the solar system, and are an excellent indication that Lutetia formed in the inner solar system, in the same region where the Earth did.

So Lutetia is a local! There aren't many like it in the asteroid main belt between Mars and Jupiter, and in fact it's a bit of a mystery how it got there; perhaps a near encounter with Earth or Venus flung it out that way, and then the influence of Jupiter made its orbit circular. And there it sits, a relatively pristine example of what the solar system was like when it was young. Currently, the Dawn space mission is orbiting the large asteroid Vesta, and will make its way to Ceres, the largest asteroid, after that. I have to wonder if NASA is eyeing Lutetia as another possible target. It's an amazing chance to visit an object that may yield a lot of insight into our own planet when it was but a youth.

After all, you can take the asteroid out of the inner solar system, but you can't take the inner solar system out of the asteroid.

This image, taken at closest approach, shows how battered and worn Lutetia is. Craters pockmark the surface, including several that are many kilometers across. Like the Martian moon Phobos, grooves line the surface, which may be from boulders rolling around, perhaps ejected from some of the craters when they were formed. They may alternatively be stress fractures from impacts; there is still a lively debate over what causes these features in small bodies.

Much of the surface appears smooth, indicating great age for this object. Over billions of years it's been assaulted by dust grains moving at incredible speeds, as well as the solar wind. This has essentially sandblasted the surface, taking - literally - the edge off of the rims of craters. We have very few high-resolution images of asteroids, and the more we get, the more we learn about them. Given that every now and again we get hit by them, I'm a big fan of understanding them better.

For the first time ever, a spacecraft approached closely enough to the asteroid Lutetia to see its surface clearly. Craters dot the surface, as well as grooves. Note the elongated crater near the bottom (left of center); was that from a nearly horizontal impact? It's curious that it points almost directly to the crater to the left. That may just be coincidence; the surface is so cratered that some are bound to be in patterns just randomly.Credit: ESA 2010 MPS for OSIRIS Team. MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Lutetia Closeup 2

Another closeup of Lutetia's surface provided by Rosetta. In this shot, you can again see a variety of craters peppering the asteroid, as well as some grooves that follow the landscape. Those curves give a relative age for the grooves: they must have formed after the impact crater on the right, which distorted the landscape. Also, had they formed before, the impact would have eradicated them. Images like this can give scientists a vast amount of insight into the history of the asteroid.Credit: ESA 2010 MPS for OSIRIS Team. MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Crescent asteroid

After Rosetta passed Lutetia, its cameras were pointed back to the rock, and therefore back toward the inner solar system. That geometry gives us an amazing, brooding, and lovely view we never get from Earth: a crescent asteroid.

When Rosetta was still 36,000 km (22,000 miles) from Lutetia, it snapped this jaw-dropping shot of the asteroid with Saturn in the distant background. This means the spacecraft, the asteroid, and Saturn were almost exactly along the same line, a configuration that probably only lasted for a few seconds. It's remarkable that controllers on the ground were able to take this picture at just the right moment to obtain this amazing picture!